Recently, electrical apparatuses such as a rechargeable power tool, in which an electric power is supplied from a secondary battery in a battery pack to a motor, are widely used. Such a rechargeable electrical apparatus using a secondary battery includes an apparatus main body such as an power tool main body; and a battery pack, installed in the apparatus main body, having a secondary battery for supplying an electric power to drive the apparatus main body. Conventionally, the secondary battery in the battery pack has a plurality of battery cells, i.e., unit cells, serially connected to one another, and a voltage of each battery cell is monitored. Thus, when at least one battery cell has a voltage lower than a preset voltage level, or at least one voltage difference between the battery cells is greater than a preset voltage difference level, a protection process is executed for, e.g., preventing the use of the secondary battery (see, for example, Japanese Laid-Open Application No. H10-27630).
However, in the conventional battery-driven electrical apparatus described above, since the voltages of the respective battery cells in the secondary battery have to be constantly monitored and a protection circuit for executing the protection process has to be kept activated, an idling current needs to flow in the protection circuit even while the electrical apparatus is not in operation. Thus, even while the apparatus main body is not in operation, a power has to be constantly consumed, though in a very small amount. Therefore, the electric charge in the secondary battery is kept being dissipated, which may even lead to a complete discharge in the secondary battery for a certain type of the secondary battery to thereby deteriorate the performance of the secondary battery, e.g., due to a decrease in the number of recharging cycles or make the apparatus main body unavailable even when it is needed to be operated.